The invention relates to a method for producing surfactant-templated thin films and more particularly, to an evaporation-induced method for producing surfactant-templated hybrid (inorganic/organic) and composite thin films.
Hybrid organic/inorganic films with controlled pore structure and surface chemistry are of interest for a wide range of applications including membranes, sensors, low dielectric constant (low k) films, photonic materials, and optical hosts. Surfactant templating is a rather recent approach toward achieving pore size control of inorganic frameworks, and so-called hybrid sol-gel chemistry provides a convenient route to derivatize the pore surfaces with covalently-bonded organic ligands. Burkett et al. (Burkett, S., Sims, S., and Mann, S., Chemical Communications, 1996, 11, 1367-1368), Fowler et al. (Fowler, C., Burkett, S., and Mann, S., Chemical Communications, 1997, 1769-1770), and Lim et al. (Lim, M., Blanford, C., and Stein, A., J. Amer. Chem. Soc., 1997, 119, 4090-4091) recently combined these approaches to form hybrid inorganic/organic mesoporous silica. Their synthesis procedures involved reacting tetraalkoxysilanes (Si(OR)4, where R=ethyl or methyl) and an organoalkoxysilane (Rxe2x80x2Si(OR)3, where Rxe2x80x2 is a non-hydrolyzable organic ligand) with water under basic pH conditions in the presence of surfactant (cetyltrimethylammonium bromide) with initial surfactant concentration co greater than cmc. These procedures result in the precipitation of powder. Various acid/solvent extraction procedures were used to remove the surfactant, resulting in organically-modified mesoporous powders with 1-dimensional, hexagonal architectures. The organic ligands in the mesoporous products included vinyl, phenyl, n-octyl, 3-sulfanylpropyl, aminopropyl, 2,3-epoxypropoxy, and imidazole. In all of these examples, the hybrid mesoporous silica was in the form of powder, precluding its use in such promising applications as membranes, low k films, and optically-based sensors that generally require transparent, defect-free, supported thin films.
A second general approach to preparing hybrid-mesophases is to prepare a stable, mesoporous silica product and then to react the pore surfaces with various organic groups using standard silane coupling chemistry. For example, Feng et al. (Feng, X., Fryxell, G. Wang, L. Kim. A., Liu, J. and Kemner, K., Science, 1997, 276, 923-926) prepared mesoporous silica products using cetyltrimethylammonium chloride as the surfactant template. After calcination, the mesoporous silica was reacted with trimethoxymercaptopropylsilane. The powder was used to remove mercury and other heavy metals from contaminated solutions. Mesoporous silicas have also been organically-derivatized via vapor phase techniques. These powders suffer the same limitations as described above.
Sellinger et al. (Sellinger, A., Weiss, P., Nguyen, A., Lu, Y., Assink, R., Gong, W., and Brinker, C., Nature, 1998, 394, 256-260; incorporated herein by reference) describe a solvent evaporation technique to form ordered structures through a liquid phase process, but with little or no porosity. Brinker et al. (U.S. Pat. No. 5,858,457, issued on Jan. 12, 1999) describe a solvent-evaporation method to form mesostructured films using metal oxides but the method does not provide for the preparation of hybrid inorganic/organic and composite thin films. Brinker et al. also do not provide for covalently bonding ligands to the porous film structure or for entrapping molecules within the pores. Useful would be a liquid-phase method to form porous thin films with inorganic or organic functional groups using a solvent evaporation technique with essentially uni-modal pore size distributions and high surface areas.
According to the present invention, an evaporation-induced self-assembly method is provided to prepare a porous, hybrid surfactant-templated, thin film by mixing a silica sol precursor, a solvent, a surfactant, and an interstitial compound to first form a silica sol, evaporating a portion of the solvent to form a liquid crystalline thin film mesophase and heating the liquid crystalline mesophase to remove surfactant. Because the surfactant is at a concentration less than the critical micelle concentration, evaporation of a portion of the solvent, such as can occur during coating onto a substrate or during aerosol processing or spray drying, forms a liquid-phase crystalline mesophase material. Coating onto a substrate by spin-coating, dip-coating and spray-coating forms a thin film wherein the thin film can be either ordered or disordered. Processing by aerosol processing or spray drying permits formation of structured particles.
The interstitial compound can be inorganic or organic, and more particularly is an organoalkoxysilanes, proteins, dyes, or metal-containing compounds. These interstitial molecules are covalently bonded or physically entrapped within the mesoporous structure. The formed thin film has pores of diameter of approximately less than 200 xc3x85 with a surface area greater than approximately 100 cm2/cm2 of deposited film.